The present invention relates to a spring-type brake actuator for the braking system for a vehicle, and in particular to a piston configuration for such an actuator.
It is well known to employ so-called “spring brake” actuators to provide service, parking and emergency brake operation on vehicles such as commercial trucks, tractors and trailers equipped with lever-operated drum or disc brakes. Spring-type brake actuators are typically pneumatically operated, and are supplied with operating air from a compressed air source on the vehicle. These actuators also typically are arranged in a “fail-safe” manner, i.e., where the actuator defaults to a brake application state upon loss of operating air pressure.
An example prior art spring brake actuator is shown in cross-section view in FIG. 1. Actuator housing 1 includes a rear cylinder 2 in which a rear piston 3 is displaceably arranged. The inner wall of the rear cylinder and a chamber-side of the rear piston define a rear ventilation chamber 4. The other side of the rear piston bears on a brake actuator spring 5. This spring is also known in the art as a “power spring” or a “parking brake spring,” and these terms may be used interchangeably.
The rear ventilation chamber is isolated from the spring side of piston 3 by an annular seal 6. An intermediate flange 8 (also known as a “wall”) separates rear cylinder 2 from a front cylinder 9. The intermediate flange 8 traversed by a seal 10 through which passes a parking brake application rod 11, formed as an extension of rear piston 3. The parking brake application rod 11 can be displaced in the intermediate flange 8 by the rear piston. A front ventilation chamber 7 within front cylinder 9 is delimited by the cylinder inner wall and a front piston 13 and annular diaphragm 14. The rear piston 3 and the front piston 13 are in non-coupled contact with one another by means of the parking brake application rod 11, such that the front piston 13 can be displaced in a brake application direction by the rear piston 3 and/or by the application of pneumatic pressure in front ventilation chamber 7. An actuating rod 15 for actuating a brake lever of a vehicle brake is provided on the front side of the front piston 13.
FIG. 1 also shows mounting studs 16 provided for mounting of the actuator 1 on the vehicle brake, as well as a light return spring 18 which biases front piston 13 toward the rear of front chamber 7.
When no pneumatic pressure is present in the FIG. 1 actuator unit, the brake actuation spring 5 applies a high spring force to rear piston 3, which in turn applies this force via parking brake application rod 11 to front piston 13 to cause the actuator rod 15 to apply the vehicle brake. In this state, the vehicle brake functions as a parking brake, preventing vehicle movement.
When release of the parking brake is desired, the rear ventilation chamber 4 is filled with compressed air via a ventilation port (not illustrated). As the force generated by the increasing air pressure on the front side of rear piston 3 exceeds the force generated by brake application spring 5, the rear piston 3 and parking brake application rod 11 move toward the rear of the rear cylinder 2, compressing spring 5 and causing air in the rear of rear cylinder 2 to be vented to atmosphere through passages in rear piston 3 (not illustrated) to vent path 19.
As parking brake application rod 11 moves towards the rear, the force previously applied to front piston 13 is relieved, and the return spring 18 biases the front piston 13 toward the rear of front cylinder 9, thereby withdrawing actuating rod 15 away from and releasing the vehicle brake. The vehicle therefore moves from a state in which it is braked by the brake actuator spring 5, to a non-braked state in which the vehicle may be moved. The vehicle brake is applied as a service during normal operation by admitting compressed air into the front ventilation chamber 7 (via a port not shown in FIG. 1). Because air pressure in rear ventilation chamber 4 continues to hold parking brake application rod 11 at the rear of the rear cylinder 2, the front piston 13 and actuating rod 15 are free to move forward and backward within the front cylinder as necessary to respond to the operator's brake actuation demands.
The parking brake piston in a spring brake actuator (in the FIG. 1 example, rear piston 3) may be a relatively complex structure, and therefore typically is formed in one piece as a cast part. The cast piston is most commonly cast from Aluminum, a material with desirable light weight and suitability for using in casting processes.
Although cast aluminum parking brake pistons are relatively low cost, they are subject to large fluctuations in cost due to large Aluminum material market price fluctuations. Moreover, the tooling used for forming cast Aluminum pistons tends to be relatively short-lived, on the order of only 100,000 casting shots before the tooling must be replaced. This short tooling life raises the cost of the case Aluminum piston parts over which the tooling costs are distributed. Cast Aluminum parking brake pistons also typically require costly machining operations to prepare their surfaces for use with rubber diaphragms, as well as for receiving parking brake release rods and finishing of ventilation features.
In view of the foregoing problems with current spring-type brake actuator parking brake pistons and related actuator components, the present invention provides an improved parking brake piston and spring brake actuator which, as compared to prior art cast parking brake pistons, is simpler and less costly to manufacture and assemble, reduces stresses on the actuator housing when the parking brake piston is manually withdrawn with a parking brake release tool, and allows the use of self-ventilating chambers and thus eliminate the need to have external breathing and its associated corrosion problems.
In the present invention the previous cast aluminum parking brake piston is replaced by a stamped piston arrangement, in which a stamped disk is affixed to the parking brake actuator diaphragm, preferably with the diaphragm captured between opposing stamped plates. The use of stamping permits the use of stamping tooling which is much longer-lived than cast tooling (typically on the order of one million stampings before tooling replacement, as compared to typically on the order of 100,000 casting shots before the casting tooling must be replaced). Preferably, the stamped piston design includes a threaded insert which secures the parking brake piston to the parking brake retraction rod, a feature which on an equivalent cast aluminum parking brake piston tooling would require expensive side features to be included with the casting die. The present approach thus provides an economical way to provide for manual retraction of the parking brake actuator in a manner already familiar to vehicle operators and technicians. The threaded insert may be formed or attached to the stamped piston in a variety of ways, for example, by spot-welding to the stamped piston. Further, a pressed-in threaded insert may be used with the stampings, as such an insert permits the use of a larger diameter breather valve arrangement than in a cast parking brake piston.
In a preferred embodiment, the stamped piston is paired with a spring seat cap element, also preferably formed by stamping, which abuts the stamped piston on the power spring side of the piston. The spring seat cap element further may be arranged to receive the brake-side end of the power spring, such that the force of the spring is borne by the spring seat cap element instead of being directly applied to the stamped piston plate. This configuration permits the use of a stamped piston plate and a spring seat cap element with thicknesses which are individually insufficient to sustain the axial force generated by the power spring, but when combined are sufficient to successfully manage the force of the power spring as a result of the effective doubling of the thickness of these stamped components in the region of the brake-side end of the power spring. This permits the use of unexpectedly thin, less costly parking brake release piston components (less costly in terms of both material costs and production costs (e.g., lower tooling costs due to use of thinner material sheets which do not require high-force stamping presses and which do not wear tooling as quickly as thicker materials), while still providing sufficient strength to bear the loads applied to the parking brake release piston by the power spring.
Because no significant tension loads must be carried through the spring seat cap element and the stamped piston plate, there is no need to provide a strong bonding between these components, such as a full circumferential weld of the cap to the piston. As a result, costly and time consuming rig high tensile strength joining methods, such as full peripheral welding, riveting, or use of fasteners may be eliminated. In some applications, it may be desirable to lightly spot-weld the spring seat cap element to the stamped piston to aid actuator assembly by ensuring concentric arrangements of these components. Alternatively, because these components are only loaded in compression relative to one another, in some applications the fixing of the spring seat cap element to the stamped piston plate may be dispensed with altogether. It is further advantageous if the spring seat cap element has a generally conical shape above the stamped piston, an arrangement which provides for greater resistance against deformation in tension or compression than a cylindrically-configured cap element (for example, when a manual brake retraction tool pulls on the cap element, or when the top of the cap element serves as a piston travel stop against the rear of the power spring chamber).
The design also may include a diaphragm-to-rod sealing element and an internal breathing arrangement within the stamped parking brake piston center arrangements to permit chamber breathing, eliminating any need to allow outside environmental air to enter the power spring portion of the spring housing and thereby eliminating the potential for contaminant entry into the brake actuator housing. A stamped spring seat cap element also provides for a larger internal volume under the cap for elements such as breather valves than was typically available in prior art cast parking brake release piston designs.
In a further embodiment, the parking brake retraction rod passes through an intermediate flange of the brake actuator, and has a plate or equivalent structure affixed to the end of the rod on the side of the intermediate flange opposite the parking brake actuator. With such an arrangement, once sufficient pressure is present to fully retract the parking brake piston to the point that the plate at the end of the rod abuts the intermediate flange of the actuator housing, (for example, at approximately 70-75 psi), any further increases in pressure (for example, an increase to a system pressure of 135 psi) results in the additional pressure load advantageously being carried by the intermediate flange, rather than the parking brake section's portion of the housing (also known as the spring housing) as is typical with prior art conventional brake actuators, whose parking brake pistons apply all of their axial loads through the compressed power spring to the end of the housing. Thus, the present invention permits design of the parking brake housing portion of the actuator to significantly lower stress levels, enabling further weight and cost reductions as compared to prior art actuators.
The material used for the stamped piston may be a naturally corrosion-resistant material such as aluminum, or may be a material such as steel to which a corrosion-inhibiting coating is applied. In view of the decreased exposure to external environmental contaminants in the present invention, additional corrosion-inhibiting coatings may be dispensed with altogether.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.